In recovery of gold from gold ores by the cyanide extraction process, it is common for a portion of the gold to be associated with sulfide and arsenide mineral particles and thus not be available for dissolution in aqueous cyanide solution. At least as to sulfides, it has been known as early as the 1930's as evidenced by the practice of Lakeshore Mines Company in Canada, to separate sulfide mineral particles from the bulk of the gold ore by flotation. The sulfide mineral particles were then roasted to oxidize the mineral and break up the mineral crystal structure of the sulfide. The calcine resulting from roasting was then subjected to leaching in aqueous cyanide to recover gold. Generally a preponderant portion of gold and other cyanide leachable metals locked up in sulfidic and arsenide crystal structures can be recovered in this manner.
The roasting of sulfides and arsenides as previously practiced has certain difficulties. The roasting process produces sulfur dioxide and arsenic oxide as products which comprise or are entrained in flue gas. If flue gases containing these oxides are not treated to remove them prior to release to the atmosphere, a severe pollution problem exists. As a practical matter, such as operation releasing these oxides to the atmosphere could not likely obtain an operating permit in most localities. Pollution control apparatus for treating such flue gas to remove such oxides is expensive both with respect to initial capital outlet and operating costs. Further, one can expect a significant ongoing corrosion problem when treating such a flue gas.